The present invention relates to a pneumatic tire, more particularly to a bead structure suitable for heavy duty tires such as truck/bus tires which is capable of improving the bead durability.
In recent years, as a carcass securing structure suitable for heavy duty tires, a carcass ply (a1) of which edge portions (a2) are wound almost once around the bead cores (b) as shown in FIG. 5 has been proposed as disclosed in Japanese patent application publication No. JP-A-2005-162057 (also disclosed in family members US2005/0081976A1 and CN1605481A).
This structure contributes to a remarkable tire weight reduction and also has advantages that bead failures such as cord loosening and ply separation which are liable to occur starting from the terminal end of the carcass ply can be prevented because the terminal end (a2e) of the wound portion (a2) is positioned in a relatively stable region near the bead core (b).
On the other hand, as the carcass ply is wound from the axially inside to the outside of the tire, on the axially inside of the bead core (b), the carcass ply (a1) is pulled radially outwards as the tire is inflated. In the case of heavy duty tires, such tire inflation pressure is very high-usually 600 to 800 kPa—in comparison with passenger car tires, and the radially outward tension (F) tends to rotate the bead core (b) as indicated in FIG. 5 by an arrow M. Therefore, if the bead core (b) is rotated, the bead toe (ct) is lifted from the bead seat (d) of the wheel rim, accompanying the radially outward movement of the carcass ply. Similarly, if the arrangement of bead wires constituting the bead core (b) gets out of order in its axially inside portion, the bead toe (ct) is lifted. Thus, the bead durability is further deteriorated.
If this state, namely the bead toe (ct) is lifted, continues for a long time, the bead toe (ct) is very liable to be deformed permanently. Therefore, if the tire is once removed from the rim, it is difficult to remount the tire because a high percentage of the air injected into the tire to inflate the tire and to place the bead portions in place leaks through between the bead bottom and bead seat. Even if well remounted, there is a possibility of bead unseating under very severe service conditions and of deterioration of durability.